ath10k: disable irqs after fw crash

It makes little sense to keep handling irqs if fw
is dead.

This prevents multiple fw register dumps upon
crash on some devices (seen on QCA6174).

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>